The invention relates to a horn antenna arrangement comprising an H-plane sectoral horn wherein with reference to a cylindrical co-ordinate system having a rectilinear z-axis which is normal to a reference plane parallel to the H-plane, the sectoral horn has a wide angle of flare about the z-axis in the reference plane, said angle of flare being not greater than 360 degrees, the sectoral horn being bounded over the whole of said angle of flare by conductive surfaces spaced apart in the z-direction and conductively connected to conductive planar side surfaces arranged radially to the z-axis at each end of the angle of flare, and wherein the aperture of the horn substantially conforms to a notional surface which is cylindrical about the z-axis, in combination with a feeder waveguide formed between substantially orthogonally disposed first and second pairs of parallel spaced conductive surfaces, said feeder waveguide extending from the throat of the sectoral horn and being provided with launching means for launching radio-frequency energy along said feeder waveguide towards said horn substantially only in a fundamental mode over an operating frequency range. Since an antenna is reciprocal in nature it is to be understood that the feeder waveguide can additionally or alternatively receive microwave energy from the throat of the horn in substantially only said fundamental mode over the operating frequency range.
Such an antenna may be used in a broad-band direction-finding system comprising a set of N adjacent similar such antennas whose respective main beam axes are spaced at regular angular intervals of (360/N) degrees (normally in azimuth). An R.F. source whose direction relative to the system is to be found may be detected by summing the output signals of all the antennas, and said direction may be established by comparing the magnitudes of the output signals of a suitable pair of adjacent antennas of the set. In order to provide substantially the same probability of detection of an R.F. source for all angles in azimuth and in order to provide optimum accuracy in establishing the direction of the source, it is desirable that the power level of an antenna main beam (relative to its peak level) in a direction corresponding to the main beam axis of an adjacent antenna, i.e. at an angle of plus or minus (360/N) degrees to its own main beam axis, should lie approximately in the range of -8 dB to -15 dB over the operating frequency range of the system.
An antenna as set forth in the first paragraph of this specification is disclosed in British Patent GB 2 090 068 B. In that antenna, electromagnetic energy is launched into the horn towards the aperture (or mouth) of the horn by a rectangular waveguide having a pair of opposed E-plane ridges. In order to obtain a substantially constant beamwidth over an operating frequency range of 3:1 which includes a band of frequencies immediately above the cut-off frequency of the TE(3,0)mode, the ridges are spaced along the waveguide from the throat of the horn: in practice, the generation of the TE(3,O) mode by the ridged waveguide is adjusted on test to be so phased with respect to the horn as to minimise variations of beamwidth with frequency in said band immediately above the TE(3,O) cut-off frequency. Without this phasing correction the higher order modes which are generated by the abrupt transition from the rectangular waveguide feed at the throat of the horn, will also be radiated and the phase relation between these higher order modes and the fundamental mode will vary with frequency. This generally results in a radiation pattern which varies greatly with frequency. The aforementioned phasing correction attempts to overcome these variations by at least partial cancellation and results in some reduction in beamwidth variation.
It is an object of the invention to provide an improved sectoral horn antenna arrangement in which the sectoral horn can be fed from a feeder waveguide so that excitation and radiation of higher modes can be substantially reduced and variations in beam width with frequency can be reduced.